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Modes Of Heat Transfer

 

Heat Transfer takes place from hot body to cold body. There are basically three modes of heat transfer by which heat can be transferred from hot body to cold body: Conduction, Convection & radiation. Any type of heat transfer from one body to another occurs by the combination of these three heat transfer modes. Heat transfer is one of very important aspect of product design.

 

 

 

 


Conductive heat Transfer

Conduction heat transfer is the transfer of heat between two or more bodies that are in direct contact with each other. In figure-1, transfer of heat from Face A to Face B is conductive heat transfer considering T2<T1.

Rate of conductive heat transfer is directly proportional to contact area, thermal conductivity of the material & temperature difference and inversely proportional to thickness of the material.

Conductive Heat transfer (Q cd) = -K A dT / t

Q cd = Convective heat transfer per unit time in watt

A  = heat transfer area in m2

k = Thermal Conductivity of the material 

dT = Difference in temperature of hot and cold body

t = Thickness of a product where heat need to be transferred.

 


Convective heat Transfer

Transfer of heat from hot body to cold body by the movement of fluids (Air/liquid). Convective heat transfer plays very important role in cooling/heating of a body. In figure-1 transfer of heat from Face B to air is convective heat transfer considering T4<T2 . 

Types of Convective Heat Transfer

  • Forced Convection
  • Natural Convection

Natural Convection

Natural convection is caused by density variation of fluid (Buoyancy Forces) that is caused by temperature variations in the fluid. Hot fluid/air rise up and is replaced by cooler fluid/air that will also heat and rise. This phenomenon is known as natural convection.

Forced Convection

Forced convection heat transfer occurs when flow of fluid/air is caused by an external force such as fan.

Convection Heat transfer (Q c) = hc A dT        

Q c = Convective heat transfer per unit time in Watt

A = heat transfer area in m2

hc= Convective Heat Transfer Coefficient (W/m2K or W/m2oC)

dT = Difference in Temperature of hot and cold body

Heat Transfer Coefficient

Value of convective heat transfer coefficient (hc) depends on the type of media (gas or liquid), Flow velocity, and temperature

Generally in electronics cooling air is used as heat transfer medium. Value of convective heat transfer coefficient for air in Free Convection varies from 5 - 25 (W/m2K) and with forced convection it varies from 10 - 200 (W/m2K)

Calculation of value of Convective heat transfer coefficient of Air

hc = 10.45 - v + 10 v1/2

v = Relative speed of the object through the air (in the range of 2 to 20 m/s)

 


Radiation Heat Transfer:

When two bodies are at different temperature are at a distance, Transfer of heat from higher temperature body to lower temperature by thermal radiations is known as radiation heat transfer. In figure-1 transfer of heat from Solid- Body-1 to Solid-Body-2 is radiation heat transfer.

Radiation heat transfer occurs due to electromagnetic waves, it does not require any medium to transfer heat. Radiation heat transfer can happen in vacuum also. Best example of radiation heat transfer is, transfer of heat from sun to earth.

The radiation energy per unit time is proportional to the fourth power of the Temperature

Radiation Heat Transfer per unit time (Qr) = ε σ T4 A

σ (Stefan-Boltzmann Constant) = 5.6703 10-8 (W/m2K4) 

T = absolute temperature in Kelvin

A = area of the emitting body in m2

ε = emissivity coefficient of the body, Value of emissivity for black body is “one”

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